The use of combinations of organic compounds having at least two intramolecular carboxylic anhydride groups per molecule, in particular of copolymers containing succinic anhydride and polyhydroxyl compounds as binders for paints and coating compounds has already been disclosed in EP-A-48,128. The idea of applying the principle of this prior art publication to the reaction of amines with anhydrides fails, however, due to the fact that the reaction of amines with anhydrides is a reaction which proceeds very quickly even at room temperature and leads to crosslinked products with the cleavage of anhydrides. The extremely short pot lives resulting therefrom have until now prevented the concomitant use of polyanhydrides and polyamines in coating systems.
A possible solution is described in DE-OS (German Published Specification) 2,853,477. This publication describes mixtures of blocked polyamines and polyanhydrides which have good pot lives but are cured upon addition of water to give crosslinked products. The suitable blocked polyamines described are ketimines or aldimines which are obtained by reaction of polyamines with ketones or aldehydes.
The polyanhydrides mentioned are compounds having at least 2 cyclic carboxylic anhydride groups in the molecule, in particular reaction products of polyols with cyclic dianhydrides in a hydroxyl/anyhdride group ratio of 1:2, or copolymers of an unsaturated cyclic anhydride with poly-unsaturated compounds or .alpha.-olefins.
The olefinically unsaturated compounds to be used in the preparation of the copolymers are deal with in the general description of DE-OS (German Published Specification) 2,853,477 only in a very generalized manner; in particular no mention is made of the ratios of amounts of the individual monomers to be used in the preparation of the copolymers. The concrete disclosure of the exemplary embodiments is limited to copolymers of butadiene oil and maleic anhydride in a ratio of 1:1 and to copolymers of tung oil with maleic anhydride. However, these copolymers have disadvantages, since their combination with bisketimines or bisaldimines eventually lead to strongly discoloured products. Besides coating agents containing unsaturated oils such as butadiene oil or tung oil as binder component give coatings which have a strong tendency to embrittle and are not weatherproof.
In addition, as seen in the exemplary embodiments of DE-OS (German Published Specification) 2,853,477, dimethylformamide has to be used as solvent for the processing of the binders described specifically in this publication, in which also unacceptable solids contents of merely about 20% are used.
A further possibility of increasing the pot lives is the use oxazolanes instead of amines.
The principle of water-curable compositions of oxazolanes and polyanhydrides are known from DE-OS (German Published Specification) 2,610,406. In this publication, oxazolanes are combined with polyanhydrides for water-curable sealing and adhesive substances. Polyanhydrides described as suitable are reaction products of poly-unsaturated fatty acids with maleic anhydride and polyanhydride of C.sub.3 -C.sub.6 -alkyl (meth)acrylate and maleic anhydride, in particular butyl acrylate and maleic anhydride.
Furthermore, in-house experiments by the applicants have shown that the systems specifically described in DE-OS (German Published Specification) 2,610,406 are still very much in need of improvement with respect to the suitability for the preparation of high-quality, colourless paint films of high hardness and good resistance to solvents and chemicals. This is true not only for the systems described in the exemplary embodiments and based on copolymers of maleic anhydride and butyl acrylate but also for the systems based on the reaction products of maleic anhydride with polyunsaturated fatty acid esters, which lead to final products which become yellow.
Accordingly, the object of the invention was to provide novel binder combinations based on polyanhydrides and modified polyamines which are suitable for the preparation of high-quality systems which are curable, if necessary under the influence of air moisture, in which the binder systems have a sufficient pot life and are curable within an acceptable period of time to give clear and solvent-resistant films.